The present invention relates to an improved electrophotographic photoconductor comprising a substantially transparent substrate, an electroconductive metal layer formed thereon and a photoconductive layer formed on the electroconductive metal layer.
Electrophotographic photoconductors of the type comprising a substrate made of, for example, a plastic film, an electroconductive metal layer formed thereon by vacuum evaporation or sputtering of an electroconductive metal, and a photoconductive layer formed on the electroconductive metal layer are widely used at present.
As such plastic film, a polyethylene terephthalate film (hereinafter referred to as "PET film") is most widely used because it is advantageous in strength, durability, smoothness and dimensional stability over other plastic film materials.
As the material for the electroconductive layer, aluminum is usually employed. A PET film coated with aluminum is mass-produced and commercially available.
The reasons why aluminum is most widely used as the material for the electroconductive layer of the conventional electrophotographic photoconductors are that (1) a thin film of aluminum can be formed on a plastic film relatively without difficulty, (2) non-ohmic contact is easily attained at the interface of an electroconductive layer made of aluminum and a photoconductive layer formed thereon, (3) when an electroconductive layer made of aluminum is employed, an electrophotographic photoconductor which is excellent in the fundamental electrophotographic properties, such as charge acceptance, charge retention, dark decay, photosensitivity and residual potential, can be obtained, and (4) aluminum itself is an inexpensive material.
As a metal other than aluminum with which a PET film is coated, Ni can be used. In addition to this, Ti, Cr, Co and W can also be employed. These metals, however, are inferior to aluminum in the charge acceptance and charge retention, when used in the electrophotographic photoconductor. Furthermore, these metals have the shortcoming that the durability is poor. With respect to the durability, aluminum also has the same shortcoming as will be explained in detail later.
As the materials for the photoconductive layer, a variety of materials are employed. The main component of the photoconductive layer is a photoelectrically active semiconductor. Representative examples of such material are inorganic materials such as selenium, selenium alloys, CdS and ZnO and organic polymers and organic pigments such as polyvinylcarbazole and phthalocyanine.
As the substrate for the photoconductive layer, an electroconductive plastic film, for instance, an aluminum-coated PET film, is in general use. In addition to this, a Ni-coated plastic film and Ti-coated plastic film can also be employed. When the aluminum-coated PET film is used as the substrate, an organic photoconductor made of an organic photoconductive material is used in the photoconductive layer. This is because this combination is most suitable for mass production, inexpensive and excellent in the electrophotographic properties as compared with other combinations of substrate and photoconductive layer. Further, when the above combination is used, the electrophotographic photoconductor can be formed into a sheet. This provides great freedom in designing electrophotographic copying machine.
One of the best organic electrophotographic photoconductors available at present is of the so-called function-separation type, which comprises a substrate, an electroconductive layer formed on the substrate, a charge generating layer formed on the electroconductive layer, comprising as the main component an organic pigment, and a charge transporting layer formed on the charge generating layer, comprising as the main component an organic dye serving as charge transporting material which is dispersed in a resin.
In the electrophotographic photoconductor of the above type, a positive hole transporting material is mostly used as the charge transporting material. Therefore, the above electrophotographic photoconductor is used with application of a negative charge thereto. In this electrophotographic photoconductor, aluminum, nickel or titanium is employed in the electroconductive layer. Therefore, it is unavoidable that these metals are subjected to anodic oxidation while in use, with repeated negative charging and exposure to light for formation of latent electrostatic images. More specifically, when the surface of the photoconductive layer is charged to a negative polarity, a positive charge is induced on the back side thereof on the side of the electroconductive layer. When the photoconductive layer is exposed to a light image and corresponding latent electrostatic image is formed thereon, the electric charges at the surface of the photoconductive layer dissipate through the electroconductive layer. When this is repeated over and over again, the electroconductive layer is gradually subjected to anodic oxidation. Eventually, the electroconductive layer is oxided so that the resistivity thereof highly increases, losing the function as the electroconductive layer. In particular, when the substrate is transparent and charge quenching for image transfer and cleaning is performed by exposing the photoconductive layer to light from the side of the substrate, the electroconductive layer is designed so as to be significantly thin for easy quenching. In this case, the above-mentioned oxidation of the electroconductive layer occurs very quickly.
Even when the photosensitive layer is positively charged for the formation of latent electrostatic image, the photosensitive layer is charged negatively for quenching the positive charge in order to facilitate image transfer to a transfer sheet or to clean the surface of the photoconductive layer. The above problem is unavoidable in both negative charging and positive charging.
Noble metals such as Au, Pt and Pd are of course resistant to oxidation. However, when these noble metals are employed in the electroconductive layer, a sufficiently high charge acceptance for use in practice is not obtained in the photoconductive layer and the charge retention of the photoconductive layer somehow significantly decreases during repeated use of the photoconductor. Further, these metals are too expensive to use in the electroconductive layer. Therefore, these noble metals are not suitable for use in the electroconductive layer. Ni and Ti not only have similar shortcomings to the above-mentioned shortcomings of the noble metals, but also are gradually oxidized while in repeated use.
In order to avoid the above problem, it has been proposed to interpose an intermediate layer between the electroconductive layer and the photoconductive layer. However, this is not effective for preventing the anodic oxidation of the electroconductive layer. Rather, by use of such an intermediate layer, more problems take place with respect to the performance of the electrophotographic photoconductor, with the photosensitivity being decreased and the residual potential remaining high.